IPT systems are well known for a number of industrial applications, and have particular advantages where traditional methods are unable to perform satisfactorily, for example clean rooms, people moving, materials handling and battery charging.
The basic IPT system consists of three main components, being a power supply, a primary track or coil usually consisting of an elongate conductive path and one or more pick-ups to which energy from the primary conductive path is transferred in a contactless manner. The operation of an IPT system is described in U.S. Pat. No. 5,293,308 (Boys et al), the contents of which are incorporated herein by reference.
A typical IPT system is shown in FIG. 1, in which a power supply 1 drives an elongate “track” conductor 2 with inductance LT with a constant current IT. The pick-up inductor L1 has a voltage induced in it by a fraction of the flux from the track conductor that intercepts it. This induced voltage is resonated using pick-up compensation circuitry 3, and rectified using rectifier 4 before being input into a switched-mode controller circuit 5 that produces a DC output voltage at output terminals 6 to power external loads. In most applications the pick-up compensation is a parallel capacitor that tunes L1 at the frequency of operation and the switch-mode controller operates by decoupling the pick-up in the manner described in U.S. Pat. No. 5,293,308. In these circumstances the switch-mode controller appears to be an up-converter and the input to the switch-mode controller is a DC inductor so that the rectifier acts as a simple choke input filter.
This parallel-tuned pick-up controller circuit, shown in more detail in FIG. 2, is widely used and robust. Capacitor C1 is used to tune the pick-up inductor L1 to the required frequency. The DC inductor is referenced LDC, and a filter capacitor CDC is provided across the load R. The switch S may be operated over a wide range of switching frequencies as required to control the power flow from the track to the pick-up coil for the particular application.
In operation the power taken from the track is controlled by the switch S to match the power required by the load resistor R. If the required output power is high then S is “off” for a higher percentage of the time and if it is low then S is “on” for a higher percentage of the time. In this way the power transfer from the primary conductive path to the pick-up circuit is controlled to hold the output voltage essentially constant while the load may vary. In practice the output voltage is regulated to be in the range ±10% of the required value. For voltages 10% or more high the switch is fully on while for voltages 10% or more low the switch is fully off.
In the application of the circuit there are however a number of disadvantages:                1. Even if the circuit is perfectly tuned current flowing in the pick-up coil induces a voltage back into the track conductor which is not perfectly in phase with the current in the track conductor, so that the circuit places a reactive load on the track and thence on to the power supply.        2. DC current flow in the DC inductor takes harmonic currents from the pick-up circuit and these induced harmonic voltages in the track conductor may cause EMI/RFI, and also degrade the performance. In many cases these harmonics are caused by discontinuous current flow in the DC inductor and this event causes a significant loss in power. Thus a large DC inductor is needed to prevent discontinuous current flow.        3. The reactive power in the pick-up circuit places stress on components in the pick-up circuit.        4. The DC inductor is physically large, and is an expensive component.        